This invention relates to a nonlinear resistor for all elements which require nonlinearity of resistance, and in particular, to a nonlinear resistor useful as an MIM (metal-insulator-metal) element functioning as a switching element in liquid crystal display devices, computer grade display devices or TV grade liquid crystal display devices.
The methods currently used for display of pictures in electro-optical displays, such as liquid crystal televisions are broadly classified as the simple matrix method and the active matrix method. The simple matrix method includes a liquid crystal interposed between two opposed, perpendicularly-intersecting sets of multiple ribbon-like electrodes. A drive circuit is connected to each of the ribbon-like electrodes. While the simple structure makes this method inexpensive, the resulting picture contrast is insufficient because of cross talk.
In contrast, the active matrix method utilizes switches which are adapted severally to serve individual picture elements and thus permits voltage retention. Because the selected voltage is retained even during the course of timeshared driving, the resulting large display capacity allows excellent picture qualities such as contrast. The active matrix method nevertheless has a complicated structure and so the production cost is high. For example, in manufacturing thin film transistors (TFTs), using at least five photomasks to improve the yield for superposing five or six thin films is difficult. Accordingly, the two-terminal elements which allow improved yield and reduce production costs have been used in favor of other active elements.
The metal-insulator-metal element (MIM) is representative of these two-terminal elements. The general structure and process for fabricating the MIM devices are shown in FIG. 1 and FIG. 2. The insulating films in conventional MIM elements have a lower electrode of TaO.sub.x formed by anodic oxidation. Since the specific dielectric constant is about 26, the element capacitance is as high as 0.1 pF when the size of the element is approximately 5 .mu.m.times.4 .mu.m and the anodic oxide film thickness is approximately 600 .ANG.. This element capacitance is as large as about 1/3 of the liquid crystal capacitance per picture element (200.times.200 .mu.m).
Conventional MIM elements and the method of manufacture have three disadvantages. First, in a liquid crystal panel of this quality, voltages applied to the panel are not fully apportioned to the MIM element. Consequently, the switching property is inferior since the capacity ratio of the capacitance of the liquid crystal to the MIM element is about 3. As the result, the liquid crystal panel is inferior in display quality to the TFT panel.
The second disadvantage is that the MIM element side substrate is produced by a repeated photolitho-etching step, as noted in the process flow sheet of FIG. 2. While this repetition step simplifies the production process of the MIM element as compared with that of the TFT element, it nevertheless reduces the resulting yield.
The third disadvantage of conventional MIM elements is their production inefficiency because a vacuum device is repeatedly used in the formation of the film to avoid spattering.
Accordingly, it is desirable to provide a new MIM element and method of fabricating them which overcomes the problems of the prior art outlined above.